1. Field
The present disclosure relates to a direct reforming catalyst for molten carbonate fuel cells, a method for preparing the same, and a method for improving a long-term stability of the direct reforming catalyst for molten carbonate fuel cells by controlling wettability to a molten carbonate electrolyte. A molten carbonate fuel cell using the above-mentioned direct reforming catalyst may be used widely in various systems using molten carbonate fuel cells, including large-scale distributed generation, concentration of carbon dioxide, or the like.
2. Description of the Related Art
Currently, catalysts such as nickel as a main ingredient supported on a porous inorganic carrier, such as magnesium oxide (MgO) or alumina (Al2O3) are used for direct reforming (or direct internal reforming, DIR) of a molten carbonate fuel cells. Typical examples of such catalysts that are commercially available may include those developed by Haldo-Topsoe Company (Denmark), British Gas (BG) Company (England) and Mitsubishi Electric Corporation (Japan).
Particularly, the catalyst developed by Haldo-Topsoe Company (Denmark) includes 10-40 wt % of Ni dispersed on a carrier containing MgO mixed with about 10% of Al2O3.
The catalyst developed by BG Company (England) includes Ni dispersed on a metal oxide carrier containing Al as a main ingredient mixed with Mg and Cr.
The catalyst developed by Mitsubishi Electric Corporation (Japan) includes Ni dispersed on an MgAl2O4 carrier.
However, such commercially available water vapor reforming catalysts used in internal reforming, particularly direct internal reforming, are subjected to poisoning since they are inevitably in contact with molten carbonate during the fuel cell operation. Thus, the carriers and Ni are sintered rapidly and the activity of the catalyst itself for reforming hydrocarbons into hydrogen is degraded, so that the quality and lifespan of a molten carbonate fuel cell may not be maintained at a level required for commercialization.
Due to this, many studies have been conducted to increase the lifespan of a direct reforming catalyst for molten carbonate fuel cells.
The studies according to the related art may be classified into approaches of interrupting a creepage path of electrolyte ingredients to a catalyst through improvement of catalyst packing or modification of the internal structure of a fuel cell, and approaches of developing a catalyst material having excellent anti-poisoning property against electrolyte ingredients.
With respect to the former approaches, Energy Research Corporation (ERC) (USA) have disclosed, in U.S. Pat. No. 4,467,050 (Patent Document 1), a method for manufacturing a plate-like catalyst body by forming an inorganic carrier layer on a stainless steel plate through an electrophoresis process, followed by impregnation with a catalytically active material, and mounting the plate-like catalyst body into a fuel cell. Additionally, in U.S. Pat. No. 4,788,110 (Patent Document 2), ERC have disclosed a method for reducing contact of carbonate vapor with a catalyst by mounting a structure made of a stainless steel plate between an anode and catalyst pellets, as well as a method for reducing contact of carbonate vapor with a catalyst by inserting carbonate-absorbing pellets between catalyst pellets.
In addition, Mitsubishi Electric Corporation (MELCO) (Japan) have disclosed, in U.S. Pat. No. 4,774,152 (Patent Document 3), a method for coating the surfaces of catalyst pellets with a carbonate-absorbing material comprising Al, Si and Cr as main ingredients, a method for mixing the carbonate-absorbing material with catalyst powder, or a method for placing the carbonate-absorbing material as an independent carbonate-absorbing layer on a catalyst layer.
However, although the above-mentioned methods have succeeded in increasing the lifespan of a catalyst to a certain degree, it is still insufficient as compared to 40,000 hours required for commercialization, and as well they has an additional problem of cost increase related with a manufacture of a separator caused by the modification of an internal structure.
Meanwhile, with respect to the latter approaches, in general a selection of a carrier strongly resistant to electrolyte vapor has been required so as to reinforce the anti-poisoning property of a catalyst against electrolyte ingredients. Typical examples of such carriers may include lithium aluminate or magnesia, and some catalysts including Ni supported on such carriers have been studied [Non-Patent Documents 1-4].
Particularly, Giordano et. al [Non-Patent Document 2] have disclosed that Ni in a catalyst using magnesia as a carrier are distributed well on the lattice of the carrier, and thus is more efficient as compared to Ni catalyst supported on lithium aluminate.
In addition, Paetsh and Kishida have disclosed that after Ni/MgO catalyst was used to carry out an experiment of water vapor reforming of methane in a 10-cell stack test, it was possible to increase the conversion empirically to 100% at 640° C. under a molar ratio of water vapor/methane of 2.5 [Non-Patent Document 3].
Additionally, Rostrup-Nielsen and Christiansen [Non-Patent Document 5] have disclosed that a catalyst including Ni or a noble metal ingredient, such as Ru, Rh or Pt etc., supported on MgAl2O4 carrier was used to operate a 7 kW-scale pilot plant for 3500 hours or more.
Further, recently, there have been reported applications of catalysts including ruthenium or rhodium supported on zirconia [Non-Patent Documents 6-9].
Meanwhile, Netherland Energy Research Foundation (ECN) (Netherland) and BG (England) have disclosed, in U.S. Pat. No. 4,546,091 (Patent Document 4) and U.S. Pat. No. 5,622,790 (Patent Document 5), a method for preparing a novel catalyst by supporting the Feitnecht compound containing Ni, Mg, Cr and Al on kaolin or bentonite to produce a catalyst precursor, and reported that the catalyst was highly resistant to poisoning with carbonate vapor.
In addition, ERC (Netherland) have studied a method for inhibiting carbon deposition by using Co as a cocatalyst, and BG (England) have studied a method for increasing anti-poisoning property by adding K as a cocatalyst in order to enhance reduction capability of Ni.
However, despite the above-mentioned studies, lifespan of catalysts have not yet reached to a target level required for commercialization. Moreover, in fact, precise mechanisms of catalyst poisoning have not yet been understood clearly.
In this regards, according to the recent report [Non-Patent Document 10] of FCE Company, it is reported that a catalyst is poisoned with a molten carbonate electrolyte due to an electrolyte creepage phenomenon through wet seals, or a direct reforming (or direct internal reforming, DIR) catalyst for a molten carbonate fuel cell undergoes degradation of catalytic activity by at least 70% while it is exposed to electrolyte vapor for a long time.
In other words, it is known that vapor of alkaline ingredients generated from a liquid electrolyte is mixed with fuel gas and then is in contact with a catalyst so that a large amount of alkaline ingredients may be crept to a catalyst, and then the active surface of catalyst is partially or totally covered with the alkaline ingredients, resulting in a decrease in catalytically active sites and further resulting in sintering of a carrier and nickel.
In addition to the above, sintering of a catalyst caused by an electrolyte, pore occlusion of a catalyst and dissolution of catalyst oxidation products, etc. are pointed out as the causes of degradation of catalytic activity.
In brief, in the case of a molten carbonate fuel cell, a catalyst is poisoned and loses the catalyst activity due to creepage of liquid carbonate or carbonate-related vapor (K2CO3, Li2CO3, Na2CO3 vapor or KOH, LiOH, NaOH vapor, etc.), resulting in degradation of lifespan of a whole fuel cell [Non-Patent Document 11]. As known from the above documents, some studies have been conducted to inhibit electrolyte poisoning of a direct reforming catalyst for a molten carbonate fuel cell. However, according to the related art, there have been many problems such as an incomplete interruption of carbonate creepage to a catalyst layer, or a consumption of the carbonate electrolyte in a matrix due to an introduction of a carbonate-absorbing layer which in turn makes it difficult to operate a fuel cell for a long time.